Integrating Multi-Output Power Converters Having Vertically Stacked Semiconductor Chips

ABSTRACT

A method for fabricating an electronic multi-output device. A substrate having a pad and pins is provided. A first chip is provided having a first and a second transistor integrated so that the first terminals of the transistors are merged into a common terminal on one chip surface and the patterned second and third terminals are on the opposite chip surface. The common first terminal is attached to the substrate pad. A driver and control chip is attached to the substrate pad adjacent to the first chip. The second terminals of the first and second transistors are connected by discrete first and second gang clips to respective substrate pins. A second chip is provided having a third and a fourth transistor integrated so that the second terminals of the transistors are merged into a common terminal on one chip surface. Patterned first and third terminals are on the opposite chip surface. The second chip is flipped to attach the first terminals vertically to the first and second gang clips. The third terminals are concurrently attached by discrete gang clips to respective pins. A common clip is attached to the common second terminal and connecting the common clip to a pin.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of and claims priority to U.S. patentapplication Ser. No. 14/181,966, filed on Feb. 17, 2014, which claimspriority to Provisional Application No. 61/810,860 filed on Apr. 11,2013.

FIELD

The present invention is related in general to the field ofsemiconductor devices and processes, and more specifically to thestructure and fabrication method of multi-phase half-bridges andmulti-output power switches with vertically stacked semiconductor chips,especially of different down-terminal types.

DESCRIPTION OF RELATED ART

Among the popular families of power supply circuits are the powerswitching devices for converting one DC voltage to another DC voltage.Particularly suitable for the emerging power delivery requirements arethe Power Blocks with two power MOS field effect transistors (FETs)connected in series and coupled together by a common switch node; suchassembly is also called a half bridge. When a regulating driver andcontroller is added, the assembly is referred to as Power Stage or, morecommonly, as synchronous Buck converter. In the synchronous Buckconverter, the control FET chip, also called the high-side switch, isconnected between the supply voltage V_(IN) and the LC output filter,and the synchronous (sync) FET chip, also called the low side switch, isconnected between the LC output filter and ground potential. The gatesof the control FET chip and the sync FET chip are connected to asemiconductor chip including the circuitry for the driver of theconverter and the controller; the chip is also connected to groundpotential.

For many of today's power switching devices, the chips of the powerMOSFETs and the chip of the driver and controller IC are assembledhorizontally side-by-side as individual components. Each chip istypically attached to a rectangular or square-shaped pad of a metallicleadframe; the pad is surrounded by leads as output terminals. In otherpower switching devices, the power MOSFET chips and thedriver-and-controller IC are assembled horizontally side-by-side on asingle leadframe pad, which in turn is surrounded on all four sides byleads serving as device output terminals. The leads are commonly shapedwithout cantilever extensions, and arranged in the manner of Quad FlatNo-Lead (QFN) or Small Outline No-Lead (SON) devices. The electricalconnections from the chips to the leads may be provided by bondingwires, which introduce, due to their lengths and resistances,significant parasitic inductance into the power circuit. In somerecently introduced advanced assemblies, clips substitute for manyconnecting wires. These clips are wide and introduce minimum parasiticinductance. Each assembly is typically packaged in a plasticencapsulation, and the packaged components are employed as discretebuilding blocks for board assembly of power supply systems.

In other recently introduced schemes, the control FET chip and the syncFET chip are assembled vertically on top of each other as a stack, withthe physically larger-area chip of the two attached to the leadframepad, and with clips providing the connections to the switch node and thestack top. Independent of the physical size, the sync FET chip needs alarger active area than the active area of the control FET chip, due toconsiderations of duty cycle and conduction loss. When both the syncchip and the control chip are assembled source-down, the larger (bothphysically and active area) sync chip is assembled onto the leadframepad and the smaller (both physically and active area) control chip hasits source tied to the drain of the sync chip, forming the switch node,and its drain to the input supply V_(IN); a clip is connected to theswitch node between the two chips. The pad is at ground potential andserves as a spreader of operationally generated heat; the elongated clipof the stack top is tied to input supply V_(IN).

A typical converter described in the last paragraph is depicted in FIG.1, generally designated 100. The control MOS field effect transistor(FET) 110 is stacked upon a synchronous (sync) MOSFET 120. The controlFET chip 110 of this exemplary module has a smaller area relative tosync FET chip 120. A QFN metal leadframe has a rectangular flat pad 101destined to become the heat spreader of the package; the leads 102 a and102 b are positioned in line along two opposite sides of the pad; theother pad sides may be kept free of leads. The stacking of the FET chipsis accomplished by the so-called source-down configuration: The sourceof sync FET 120 is soldered to the leadframe pad 101 by solder layer121. A first clip 140, soldered by solder layer 122 onto the drain ofsync FET 120, has the source of control FET 110 attached by solder layer111; first clip 140 thus serves as the switch node terminal of theconverter. A second clip 160 is connected by solder layer 112 to thedrain of control FET 110. Second clip 160 is attached to lead 102 b ofthe leadframe and thus connected to the input supply V_(IN). Thisconverter can operate efficiently at a frequency of 500 kHz up to 1 MHz.

SUMMARY

Applicants realized that it would be desirable for many applications(especially automotive and handheld products) to use power switches,which have only one power input yet provide multiple power outputssimultaneously. Such multi-output converters, also called multiphasehalf bridges, could be expanded to multiple power inputs and multiplepower outputs working simultaneously.

In present technology, a multiphase bridge for two output voltages needsfor each voltage channel: one packaged sync FET, one packaged controlFET, and one packaged driver/controller, thus for two channels a totalof six packaged components together with their corresponding consumptionof circuit board (PCB) real estate area. In an analysis, applicantsfound that multi-output power switches could find a wide market withmany applications if they would consume only minimal board area, incombination with low electrical impedances.

Applicants solved the problem of creating a single device producing twoor more output voltages V_(SW) from one input voltage V_(IN) while onlyconsuming minimal PCB area, when they discovered a methodology ofintegrating a first and a second vertical stack, each including acontrol n-channel FET on top of a sync n-channel FET, by merging thesync FETs into one chip of silicon so that they have only one sourceterminal on one chip surface and two or more adjacent drain terminals onthe opposite chip surface.

In the methodology, one stack of FETs is needed for each output voltageV_(SW), for example two stacks for two V_(SW). The stacking is done sothat both stacks share the electrical input and both stacks share theelectrical ground. In each stack of an exemplary embodiment of chipsfabricated with source on the bottom, the control chip is orientedsource-down on the respective V_(SW) clip, and the shared sync chip isoriented source-down on the common leadframe. For each output voltage asdetermined by the duty cycle regulated by the respectivedriver/controller chip, there is a separate V_(SW) clip. The shared syncchip has a common source attached to the common leadframe, and twoseparate drains attached to the separate V_(SW) clips. As stated, theactual values of the two different output voltages, as derived from thecommon input voltage, are determined by the duty cycles regulated by thetwo driver/controller chips.

In another exemplary embodiment having the control chip fabricated withits drain on the bottom while the sync chip is fabricated with itssource on the bottom, the control has to be flipped for assembling it ina stack. After flipping, the control source can face the clip resting onthe drain of the sync chip; extra clip segments have to be used tocontact the gate terminals of the flipped chips.

As a result of the integration, the device with multi-output powerswitches avoids altogether the parasitic impedances of PCB traces. Inaddition, the stacked power FETs offer thermal and electricalefficiencies close to the theoretical maximum. The device permits directimplementation into PCBs without the headache of first modifying afootprint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a synchronous Buck converter assembledaccording to prior art, wherein a large-area sync FET chip is attachedto a leadframe pad and topped by a small-area control FET chip; thelatter is connected by an elongated clip to leads.

FIG. 2 illustrates a perspective view of an integrated multi-outputpower converter having vertically stacked semiconductor chips accordingto the invention.

FIG. 3 shows a circuit diagram of the converter of FIG. 2 using FETsfabricated in source-down configuration; the dashed outline indicates anintegrated sync chip.

FIG. 4 shows an analogous circuit diagram of a multi-output powerconverter using FETs fabricated in drain-down configuration; the dashedoutline indicates an integrated control chip.

FIG. 5 shows a circuit diagram of a multi-output power converter usingFETs fabricated in drain-down configuration as control chips and FETsfabricated in source-down configuration as sync chips; the dashedoutlines indicate integrated chips.

FIGS. 6 to 10B depict certain process steps of assembling and packaginga multi-output power converter using FETs fabricated in drain-downconfiguration as integrated control chips and FETs fabricated insource-down configuration as integrated sync chips.

FIG. 6 illustrates the process step of attaching an integrateddriver/controller chip and an integrated sync chip onto the pad of aleadframe.

FIG. 7A shows the process step of attaching metal clips onto the syncchip and additional clips to gate leads.

FIG. 7B depicts a cross section through the assembly of FIG. 7A.

FIG. 8 illustrates the process step of flip-attaching an integratedcontrol chip onto the clips of FIG. 7A, whereby the gates of the FETsare connected to the additional clips shown in FIG. 7A.

FIG. 9 shows the process step of attaching a metal clip onto theintegrated control chip.

FIG. 10A illustrates the process step of wire-bonding the integrateddriver/controller chip to respective terminals of the sync and controlFETs; furthermore, input (V_(IN)) leads and output V_(SW1) and V_(SW2))leads are indicated.

FIG. 10B depicts a cross section through the assembly of FIG. 10A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a single electronic device creating two or more outputsfrom a single input can be found in electronic functions such asconversion and regulation, and the active electronic components employedto achieve these functions may be as diversified as MOS field effecttransistors (MOSFETs), transistors based on GaN, GaAs and other III-Vand II-IV materials, SiC, and bipolar transistors: For reasons ofexplanation and clarity, however, the following preferred embodimentfocuses on the example of a multi-output power converter.

FIG. 2 illustrates a perspective view of an embodiment of a multi-outputpower converter generally designated 200, which is shown with atransparent encapsulation 290. Preferred actual encapsulation uses ablack-colored epoxy formulation for a transfer molding technology. Theexemplary converter of FIG. 2 has a length 292 of 7 mm, and a width 293of 5 mm, and a thickness 191 of 1.0 mm. Converter 200 is constructed ona substrate, which may be a metallic leadframe or a laminate usingtapes. In all cases, the substrate has a pad for assembling (preferablyby soldering) the chips or stack of chips, and a plurality of terminalssuch as metallic leads and metal contact pads.

Visible through the transparent encapsulation is a metal leadframegenerally suitable for Quad Flat No-Lead (QFN) and Small Outline No-Lead(SON) type modules. The leadframe includes a rectangular pad 201 and aplurality of leads 202. These leads may be approximately uniformlydistributed on all four sides of the pad as in the exemplary deviceshown in FIG. 2, or they may be distributed so that one or more sides ofthe leadframe may be free of leads. Pad 201 may operate as the groundterminal of the converter, and at least one lead 203 is the electricalinput terminal V_(IN). The leadframe is preferably made of copper or acopper alloy; alternative metal selections include aluminum, iron-nickelalloys, and Kovar™. Both surfaces of the leadframe may be prepared tofacilitate solder attachment, for instance by a sequence of platedlayers of nickel, palladium, and gold. In addition, the at least onesurface may have a metal layer deposited to enhance thermalconductivity, for instance by a plated layer of silver. Preferredthickness of the leadframe metal for the exemplary embodiment shown inFIG. 2 is 0.2 mm; other embodiments may use thinner or thicker leadframemetal. From the standpoint of low cost and batch processing, it ispreferred to start with sheet metal and fabricate the leadframe as astrip by stamping or etching, and to singulate the leadframe for themodule by trimming the strip after the encapsulation process.

FIG. 2 shows a first chip 220 affixed to pad 201. For the embodimentshown in FIG. 2, chip 220 has a size of about 3.5×2.85 mm, and athickness of 0.1 mm. For other embodiments, the chip size and the chipthickness may have significantly smaller or greater values. Chip 220 hasa first and a second field-effect transistor merged so that one chipsurface (not shown in FIG. 1) has the common source terminal of thetransistors. Chips with merged or integrated transistor terminals may bereferred to herein as composite chips. The chip surface with the commonterminal is affixed to pad 201. The preferred method of affixing is bysoldering using a wafer-level solder application method such as platingor screen printing. The solder material, for instance a solder paste, isselected so that the same material can be used for all solder joints ofthe multi-output power converter, allowing a single solder reflow stepfor assembling the converter. The preferred thickness of solder layersis at least 25 μm.

On the opposite chip surface (remote from pad 201), chip 220 has a firstdrain terminal 221, belonging to the first of the merged transistors,and a second drain terminal 222, belonging to the second of the mergedtransistors. Drain terminals 221 and 222 are positioned adjacent to eachother, without scribe line between them. The first field-effecttransistor comprising the source terminal of chip 220 and first drainterminal 221 is herein referred to as Q₁; the second field-effecttransistor comprising the source terminal of chip 220 and second drainterminal 222 is herein referred to as Q₂. First drain terminal 221 isconnected by a first metal clip 241 to a first output lead 204. Seconddrain terminal 222 is connected by a second metal clip 242 to a secondoutput lead 205. The preferred metal for clips 241 and 242 is copper dueto its good electrical and thermal conductivity, suitable for powertransistors. As a result, first chip 220 is operable as the sync FETchip of power converter 200.

For the purposes of this description, the terms clip and gang clip willbe understood to refer to preformed conductive interconnects of a powertransistor module, which are attached to a power transistor after thepower transistor is singulated from a semiconductor wafer containingother units of power transistors. A clip may be provided as a piecepart, a gang chip is provided as a unit of a network held together byrails, which will be trimmed at time of singulation.

As FIG. 2 illustrates, a second chip 211 and a third chip 212 arepositioned vertically over first chip 220. Specifically, second chip 211is positioned vertically over first drain terminal 221 and third chip212 is positioned vertically over second drain terminal 222. Thesevertical chip positions are frequently referred to as stacking of chips.For the embodiment shown in FIG. 2, chips 211 and 212 each have a sizeof about 2.5×1.8 mm, and a thickness of 0.1 mm. For other embodiments,the die size and the die thickness may significantly to smaller or togreater values. Chip 211 has an FET (herein referred to as thirdtransistor Q₃) with its source terminal (not shown in FIG. 2) attachedto first clip 241, rendering clip 241 a first switch node V_(SW1), andchip 212 has an FET (herein referred to as fourth transistor Q₄) withits source terminal (not shown in FIG. 2) attached to second clip 242,rendering clip 242 a second switch node V_(SW2). The preferred method ofattaching is by soldering; preferred thickness of the solder layers isabout 25 μm. As FIG. 2 shows, first clip 241 and second clip 242 have astructure to function as respective switch nodes between the respectivedrains of sync chip 220 and the respective sources of chips 211 and 212as control chips. The clips also operate as low impedance contacts ofthe switch node to a plurality of leads 204 and 205 capable ofconducting the load current to the output inductor. Clips 241 and 242are preferably made of copper in the thickness range from about 0.2 to0.3 mm; both surfaces of the clips are preferably solderable. First clip241 and second clip 242 are preferably employed in strip form and etchedto acquire their contours and thicknesses.

The field-effect transistor comprising the source terminal of secondchip 211 and drain terminal 213 is herein referred to as Q₃; thefield-effect transistor comprising the source-terminal of third chip 212and drain terminal 214 is herein referred to as Q₄. The drain terminal213 of second chip 211 and the drain terminal 214 of third chip 212 areattached onto a common metal clip 260, referred to as third clip. Thepreferred method of attaching is by soldering with a solder layer ofabout 25 μm thickness. As FIG. 2 shows, common clip 260 has a structureto function as low impedance contact of the drain terminals of thecontrol FETs to the input supply, capable of conducting the inputcurrent. Common clip 260 is preferably made of copper in the thicknessrange from about 0.2 to 0.3 mm, more preferably about 2.5 mm. The bottomsurface of common clip 260 is solderable. Common clip 260 is preferablyemployed in strip form and etched to acquire its contours andthicknesses.

As FIG. 2 shows, common clip 260 is connected to the input lead 203 asinput terminal. As a result, second chip 211 and third chip 212 areoperable as respective control chips of converter 200. For theembodiment shown in FIG. 2, chip 220 has a size of about 3.5×2.85 mm,and a thickness of 0.1 mm. For other embodiments, the chip size and thechip thickness may have significantly smaller or greater values.

FIG. 2 depicts first chip 220 to include a first gate terminal 230 and afirst gate return terminal 231 adjacent to first drain terminal 221, andfurther a second gate terminal 232 and a second gate return 233 adjacentto second drain terminal 222. The terminals of sync FET chip 220 aremetallurgically suitable for wire bonding. In addition, second chip 211includes a gate terminal 215 adjacent to drain terminal 213, and thirdchip 212 includes a gate terminal 216 adjacent to drain terminal 214.The terminals of control FET chips 221 and 222 are metallurgicallysuitable for wire bonding.

It should be noted that the concept of component integration into onechip combined with stacking of chips can be extended to any number ofcomponents within each chip and any number of vertically stackedcomponents. Consequently, the device applications include multi-outputpower converters, regulators, applications with sensing terminals orKelvin terminals, and others.

The multi-output converter 200 further includes a firstdriver/controller chip 251 and a second driver/controller chip 252;these chips provide the driver and control functions to the converterand determine, by setting the duty cycles, the DC output voltages of theconverter relative to the DC input voltage. As examples, from an inputvoltage Of 12 v, a duty cycle of 0.5 enables an output voltage of 6 V; aduty cycle of 0.25 enables an output voltage of 3 V. Bothdriver/controller chips are attached to leadframe pad 201 and positionedin close proximity to sync chip 220. In the exemplary embodiment of FIG.2, the distance between chips 251 and 252, and chip 220 is about 300 μm.Chips 251 and 252 have back side metallization to allow attachment toleadframe pad 201 by solder (layer thickness preferably about 25 μm).The metallic solder provides high thermal conductivity for spreadingheat from chips 251 252 to pad 201. Besides soldering, conductive epoxyor silver sintering are other possible chip and clip attach techniques.In the exemplary embodiment shown in FIG. 2, chips 251 and 252 aresquare shaped (side length about 1.4 to 1.5 mm) and 0.2 mm thick. Otherembodiments may have chips, which are smaller or greater, and thicker orthinner. Still other embodiments may combine both driver/controllerchips into a single chip. Furthermore, in some converters chips 251 and252 and chips 220, 211 and 212 may have the same thickness. Firstdriver/controller chip 251 is electrically connected (for instance bybonding wires) to gate terminal 230 and gate return terminal 231, andfurther to gate terminal 215. Second driver/controller chip 251 isconnected (for instance by bonding wires) to gate terminal 232 and gatereturn terminal 233, and further to gate terminal 216.

First driver/controller chip 251 is electrically connected (for instanceby bonding wires) to gate terminal 230 and gate return terminal 231, andfurther to gate terminal 215. Second driver/controller chip 252 isconnected (for instance by bonding wires) to gate terminal 232 and gatereturn terminals 233, and further to gate terminal 216. The differencebetween the width of rectangular pad 201 and the relatively smaller sidelengths of chips 251 and 252 provides a distance between chip terminalsand leads, which may be as significant as indicated in FIG. 2. Thisdistance stretches the wire spans from chip terminals to respectiveleads such that any height difference due to the thickness of chips 251and 252 becomes insignificant. Consequently, the wire connections 253can be provided without downhill bonding, enhancing the reliability ofthe converter assembly. The preferred diameter of bonding wires 253 isabout 25 μm, but may be smaller or greater.

FIG. 3 represents a simplified circuit diagram of the embodiment of FIG.2. Field-effect transistors Q₃ (310) and Q₁ (320) are source-downtransistors. The chip of Q₃ (211 in FIG. 2) is vertically stacked ontothe chip of Q₁ (220 in FIG. 2) so that the transistors share switch nodeSW₁ (241). Field-effect transistors Q₄ (330) and Q₂ (340) aresource-down transistors. The chip of Q₄ (212 in FIG. 2) is verticallystacked onto the chip of Q₂ (220 in FIG. 2) so that the transistorsshare switch node SW₂ (242). Transistors Q₃ and Q₄ are connected (bycommon clip 260) to share the same input terminal V_(IN) (203).Transistors Q₁ and Q₂ are connected (by leadframe pad 201) to share thesame electrical ground terminal 360. As indicated by dashed outline 350,transistors Q₁ and Q₂ are integrated into the single chip 220.

An analogous circuit diagram and analogous considerations hold foranother embodiment wherein the chips are manufactured so thattransistors Q₃ (410) and Q₁ (420) as well as transistors Q₄ (430) and Q₂(440) are drain-down transistors. For this configuration, the circuitdiagram is depicted in FIG. 4. As indicated by dashed outline 451,transistors Q₃ and Q₃, both drain-down transistors, are integrated intoa single chip. This common chip is vertically placed onto the chips oftransistors Q₁ and Q₂ so that the transistors Q₃ and Q₁ can share switchnode SW₁, and the transistors Q₄ and Q₂ can share switch node SW₂.Transistors Q₃ and Q₄ are connected to share the same input terminalV_(IN) (203). Transistors Q₁ and Q₂ are connected to share the sameelectrical ground terminal 460. Since the gate terminals in transistorsQ₁ and Q₂ are located on the same side of the chip as the sourceterminal, they have to be electrically isolated from the sourceterminals. A connection between the Q₁ and Q₂ gates to their respectivepins or driver terminals can for example be realized by means ofleadframe patterning or generally package substrate (e.g. laminate)patterning. Connections to the driver can, for example, be realized bywire bonding. In an actualization of this concept, the gates oftransistors Q₁ and Q₂ face the leadframe; consequently, the leadframepad has to be designed with subdivisions to allow electrically separateconnections to the gates of transistors Q₁ and Q₂. FIG. 4 can also beused for the case of drain-down transistors with the substrate of thepackage at V_(IN).

In another embodiment of the invention, transistors Q₃ and Q₄ arefabricated as drain-down FETs while transistors Q₁ and Q₂ are fabricatedas source-down FETs. This embodiment is summarized in the simplifiedcircuit diagram of FIG. 5, and described in more detail by the assemblyprocess steps of FIGS. 6 to 9, resulting in the finished and packagedmulti-output converter illustrated by the top view of FIG. 10A and thecross section of FIG. 10B, generally designated 1000. As FIG. 5indicates, the embodiment allows the integration of transistors Q₃ andQ₄ into a single chip, indicated by outline 551, as well as theintegration of transistors Q₁ and Q₂ into another single chip, indicatedby outline 550.

The assembly method is suitable for fast and low-cost batch processing,since all attachments may be performed by a single solder material and asingle reflow temperature so that a single one-step-fits-all attachmentprocess can be employed. The solder paste is selected so that the solderreflow temperature is higher than the temperature of wire bonding.Further, downhill wire bonding is almost completely avoided; thetemperature for wire bonding is lower than the solder reflowtemperature. In addition it is preferred that piece parts, such asleadframe and clips, are provided in strip form and are only singulatedafter the encapsulation step.

The positions of parts in a 3-dimensional composite, such as a stack ofsemiconductor chips, may be described with geometrical connotations,such as “low side” or “high side”. Since the exemplary embodimentdescribed below is a multi-output power converter of the synchronousBuck converter type, the positions of parts can also be expressed infunctional connotations such as “sync” or “control”.

The first phase of the assembly method for the embodiment of FIG. 5 isdepicted in FIG. 6. The method starts with the step of providing asubstrate, in this example a metallic leadframe, which has a rectangularflat assembly pad 201 and a plurality of terminal leads 201 positionedin line with the sides of the pad. A preferred leadframe metal is copperor a copper alloy in a thickness range from about 150 to 250 μm; otheroptions include aluminum, an iron-nickel alloy, and Kovar™. Bothsurfaces of the pad have a metallurgical disposition, which facilitatessolder wetting and solderability. As an example for copper leadframes,the pad surfaces may have additional plated layers of nickel, palladiumand gold (outermost). For the leads, the surfaces facing thechips-to-be-assembled are wire bondable (preferred wire metals are goldand copper), for instance by a spot-plated layer of gold. The oppositelead surfaces are preferably solderable. Alternatively, a substrate of alaminate foil structure may be used, which has an assembly pad and aplurality of metallic terminals.

In the next step, a solder mixture is selected, preferably configured asa tin-based paste, which has a reflow temperature higher than thetemperature used for wire bonding (about 220° C.); the paste is usedthroughout the assembly.

The assembly starts by placing a driver-and-controller chip 650 on theleadframe pad 201 using a layer of solder paste with thickness about 25μm or greater; the solder paste may be dispensed on pad 201 by a syringeor by screen printing. Chip 650 has back side metallization (forinstance a layer of nickel or titanium-tungsten followed by a layer ofpalladium) in order to render the back side of semiconductor chip 650solderable.

In the next step, low side FET (sync FET) chip 220, which integrates twointegrated source-down FETs, is placed source-down adjacent todriver-and-controller chip 650 onto a layer of solder paste dispensed onpad 201. The source-down placement of FET chip 220 has the common sourceof the two integrated sync FETs face the leadframe pad. As a result, thetop view of FIG. 6 displays the drain terminals 221 and 222 of the twointegrated FETs. FIG. 6 also shows the two gate terminals 230 and 232 ofthe integrated FETs adjacent to the drain terminals 221 and 222,respectively. The preferred thickness of the assembly solder layer isabout 25 μm or greater. In order to keep parasitics as small aspossible, the distance 660 between chip 650 and chip 220 is preferablyselected as narrow as assembly layout rules allow, without risking anaccidental merging of the adjacent meniscus of the liquefied solderlayers and during the reflow process, which would be coupled withsurface tension-induced relative movements of chips 650 and 220. As anexample, for certain solder alloys and layer thicknesses, about 300 μmis a preferred distance.

FIGS. 7A and 7B illustrate the next process steps, which involve theplacing and attaching of metal clips, which are preferably provided asgang clips (low side gang clip attach). A metallic clip 241, hereincalled the first clip, and another metallic clip 242, herein called thesecond clip, are deposited onto a layers of solder paste on the drainterminals 221 and 222. First clip 241 and second clip 242 are preferablygang clips; they are shaped to serve as switching nodes of themulti-output converter; by stamping and forming or etching of thestarting metal sheet (about 0.2 to 0.3 mm thick), the clips haveobtained a structure so that clip portions get to rest on first outputlead 204 (first switching node V_(SW1)) and on second output lead 205(second switching node V_(SW2)). Cross sections of clips 241 and 242 aredepicted in FIG. 10B. With the attachment of first clip 241 and secondclip 242, first FET chip 220 is operable as the sync chip of theconverter.

FIG. 7A further shows the placing of additional metal clips, which arepreferably provided as gang clip segments. Lead 206, which is dedicatedas gate lead, has clip segment 706 attached (herein referred to asfourth clip), and lead 207, which is dedicated as another gate lead, hasclip segment 707 attached (herein referred to as fifth clip). Until thenext assembly step, the ends of both clip segments remote from therespective leads rest loosely on the insulator-protected surface of chip220. The protecting insulators may be silicon nitride, silicon dioxide,polyimide or other suitable ship passivations.

In the next assembly step, illustrated in FIG. 8, high side FET (controlFET) chip 210 is placed onto layers of solder paste dispensed on clips241 and 242, vertically stacked over sync chip 220. Chip 210 includesdual FETs; both are fabricated as drain-down FETs so that the sourceterminals and adjacent gate terminals are on one chip surface, and thedrain terminals are on the opposite chip surface. However, for theassembly depicted in FIG. 8, chip 210 is flipped so that the source andgate terminals face the sync chip 220 and the drain terminals 213 and214 are facing away from the sync chip. After flipping chip 220, therespective source terminals (not shown in FIG. 8) are aligned with clips241 and 242, covered with a layer of solder paste. When chip 210 islowered to bring the source terminals in contact with the solder layers,the gate terminals (not shown in FIG. 8) are simultaneously brought incontact with the (solder layer-covered) clips 706 and 707, respectively.As a result, leads 206 and 207 become the gate leads (high side gateleads) for the FETs of chip 210.

FIG. 9 depicts the step of attaching the common clip (high side clip,third metal clip) 260 to the drain terminals 213 and 214 of the highside (control chip) 210. Clip 260 is preferably provided as a gang clip.Third clip 260, destined to become the connector to the input supplyV_(IN) of the multi-output converter, has obtained, by stamping andforming or etching of the starting metal sheet (about 0.2 to 0.3 mmthick), a structure so that concurrently with the placement on thecontrol FET chip 210 a clip portion gets to rest on a layer of the samesolder paste dispensed on (at least one) lead 203 of the leadframe.Leads 203 serve as the input terminal of the converter, connecting tothe input supply V_(IN).

In the next process step, thermal energy is supplied to raise thetemperature for reflowing the layers of solder paste at the soldermelting temperature. The solder alloy is selected so that the solderreflow temperature is substantially higher than the temperature neededfor the wire bonding in the next process step. Since all solder layersare made of the same material, all solder connections are accomplishedconcurrently by a single reflow step. This simplifying featurecontributes significantly to a low cost of the fabrication method.

FIGS. 10A and 10B depict the next process step of wire bonding.Preferably, wires (or occasionally ribbons) of gold or copper are usedto connect the terminals of the driver-and-control chip 650 torespective leads of the leadframe, and further to the gate terminals andgate return terminals of the sync FET chip and the gate clips connectingto the gates of the control FET chip. The bonding steps avoid downhillbonding almost completely and thus minimize the well-known reliabilityissue of wire shorting during the encapsulation step.

FIG. 10B indicates the next process step of encapsulating thedriver/control chip 650, the sync FET chip 220, the control FET chip210, the metal clips, and the wire bonds in a packaging compound 290,thus creating the external dimensions of converter 1000. The bottom 201a of pad 201 and the bottoms of the leads are left un-encapsulated sothat pad bottom surface 201 a and the lead bottom surfaces can besoldered to a substrate or a circuit board which may also be a heatsink. Since the compound thickness over second clip 260 can be keptsmall, the total thickness 291 of the exemplary converter may be kept to1.5 mm. It is possible to reduce this thickness further by keeping thetop surface of second clip 260 un-encapsulated, a feature enabled by thefact that no wire bonding is involved for connecting the driver/controlFET to the drain terminals and the gate terminals of the control FET.

The overall dimensions of exemplary converter 1000 may be length 292about 7 mm and width 293 about 5 mm.

For reasons of batch processing and low fabrication cost, it ispreferred to provide the leadframe, the first clip and the second clipin strip form. In a trimming process step, the encapsulated strips aresingulated into discrete converter units like the converter depicted inFIG. 2. A preferred singulation technique is sawing.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies not only to fieldeffect transistors, but also to other suitable power transistors, tobipolar transistors, insulated gate transistors, thyristors, and others.

As another example, the above considerations for structure andfabrication method of multi-output power converters apply tomulti-output regulators, multi-output power converters, applicationswith sensing terminals, applications with Kelvin terminals, and others.

As another example, the high current capability of the multi-outputconverter can be further extended, and the efficiency further enhanced,by leaving the top surface of the second clip un-encapsulated so thatthe second clip can be connected to a heat sink, preferably bysoldering. In this configuration, the hexahedron-shaped module candissipate its heat from both large surfaces to heat sinks.

As another example, the concept of component integration into one chipcombined with stacking of chips can be extended to any number ofcomponents within each chip and any number of vertically stackedcomponents.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

We claim: 1-33. (canceled)
 34. A method for fabricating an electronicmulti-output device comprising: providing a substrate having a pad andpins; providing a first chip having a first and a second transistorintegrated so that the first terminals of the transistors are mergedinto a common terminal on one chip surface and the patterned second andthird terminals are on the opposite chip surface; attaching the commonfirst terminal to the substrate pad; attaching a driver and control chipto the substrate pad, adjacent to the first chip; connecting the secondterminals of the first and second transistors by discrete first andsecond gang clips to respective substrate pins; providing a second chiphaving a third and a fourth transistor integrated so that the secondterminals of the transistors are merged into a common terminal on onechip surface, and the patterned first and third terminals are on theopposite chip surface; flipping the second chip for attaching the firstterminals vertically to the first and second gang clips, respectively,and concurrently attaching the third terminals by discrete gang clips torespective pins; and attaching a common clip to the common secondterminal and connecting the common clip to a pin.
 35. The method ofclaim 34 further including encapsulating the chips in a packagingcompound.
 36. The method of claim 34 wherein the substrate is aleadframe.
 37. The method of claim 34 wherein the substrate is alaminate.
 38. The method of claim 34 wherein the substrate is a foil.39. The method of claim 34 wherein the first and second transistors aresource-down MOS field-effect transistors with the first terminals beingthe source terminals and the second terminals being the drain terminalsof the respective transistors; the third and fourth transistors aredrain-down MOS field-effect transistors with the second terminals beingthe drain terminals and the first terminals being the source terminalsof the respective transistors.
 40. The method of claim 34 wherein thedevice is a power converter, the substrate pad is the ground terminal,the substrate pin connected to the common clip is the input terminal,and the substrate pins connected to the first and second gang clips arethe output terminals.
 41. The method of claim 34 wherein the chips aremade of a material selected from a group including SiC, GaN, GaAs, andother III-V and II-IV compounds.